(1) The Mars probe detects rocks and soil on the surface of Mars.
Mars probe carries out in-situ rock and soil analysis on the surface of Mars, and Viking spacecraft studies the soil of Mars through X-ray fluorescence (XRF). The rover Mars Pathfinder studied the rocks and soil on the surface of Mars through the α proton X-ray spectrometer (APXS). "Opportunity" and "Spirit" rovers have been working on the surface of Mars for five years, and have carried out detailed exploration on the rocks on the surface of the landing zone. The results show that the surface of Mars is similar to desert-Gobi area, and it is composed of a large number of basalt rock fragments and fine weathered particles (Figure 28- 12). The composition of representative Martian rocks and soils is shown in Table 28-3.
Figure 28- 12 On the surface of Mars photographed by Mars Pioneer spacecraft, the gray rocks are covered with reddish-brown Martian dust.
Most of the spheres on the surface of Mars are covered with a thin layer of tiny red Martian dust, and the landing sites of Viking and Mars Pathfinder are located in these Martian dust areas. In-situ analysis of Martian soil by Viking and Mars Pathfinder obtained similar compositions (Rieder et al.,1997; Bell et al., 2000), reflecting the homogenization of sediments on the surface of Mars due to wind erosion. The composition of rocks on Mars that are not covered by Martian dust has passed the composition test of "Mars Pathfinder" and is shown as andesite (Rieder et al.,1997; McSween et al., 1999), see table 28-3 for details.
Table 28-3 Composition of Rock and Soil on Mars Surface Detected by Mars Probe
(2) Introduction of Martian rocks
Mars meteorite is the only sample of Mars rock obtained by human beings at present, which is very rare and precious. They are all magmatic accumulation rocks and basalts, including four main rock types: xonotlite, sodalite, xonotlite and plagioclase pyroxenite. Some pyroxene chondrites are basalt (B-S); The others are ultrabasic rocks (L-S lherzolite) (mcsween & treiman, 1998). Recently, meteorites with olivine phenocrysts (DaG 476, SaU 005, Dhofar 0 19, NWA 1068) have been isolated respectively. NWA 2046) and EETA 7900 1 A Figures 28- 13 and 28- 14 are appearance photos of several Martian meteorites.
Figure 28- 13 Typical Martian meteorite
Fig. 28- 14 Discovery of Martian meteorites in Antarctica (ALH8400 1)
So far, there are more than 60 unpaired meteorites from Mars, all of which are magmatic rocks without breccia (although most areas on the surface of Mars are covered with impact craters like the moon). So far, these Martian meteorites have not found a small area of sedimentary rocks like those detected in the meridian plain.
The representative SNCO Martian meteorite was discovered very early (Shergotty,1discovered in India on August 25th, 865; Nakhla, 19 1 1 was discovered in Egypt on June 28th. Chassigny,18151010 was discovered in France on October 3rd. The orthopyroxene, ALH 8400 1, 1984, was found in Allen Hill, Antarctica), but it was studied and debated for a long time to determine that it came from Mars.
As early as 1872, Tschermak recognized that the Sergo Tethys meteorite was a basalt formed under relatively oxidized conditions. Based on the young crystallization age of these SNCO meteorites (except ALH8400 1, which is about 4.0 ~ 4.5 Ga, the age of other Martian meteorites is ≤ 1.3Ga), many scholars think that they are from Mars (McSween et al.,1979a; ; Nyquist et al.,1979; Walker et al.,1979; Watson & Weatherill,1979; Wood & Ashwar,1981).1982, the discovery of the first moon meteorite made people believe that the SNCO meteorite may come from Mars. 1983, Bogard and Johnson found that EETA 7900 1 meteorite (B-S) collided with rare gases such as Ar and Ar captured in molten glass, and the isotopic composition and relative abundance of N2 and CO2 obviously matched the atmospheric abundance of Mars (Figure 28- 15, Figure 28-1. At the same time, Clayton & Mayeda (1983+0983) found that Martian meteorites formed subgroups on the oxygen isotope diagram, and the fractionation lines formed by them were obviously separated from the earth rocks and HED meteorites, which were located above and parallel to the TF line. Becker &Pepin( 1984+0984) also found that the nitrogen isotope and N/Ar ratio are consistent with the measurement results of Viking spacecraft, which further confirmed the Martian origin of these meteorites.
Fig. 28- 15 Comparison of captured gases in Mars atmosphere measured by Viking spacecraft and EETA 7900 1 glass (according to Pepin, 1985).
Fig. 28- 16 oxygen isotope composition of Martian meteorites
The main evidences that Martian meteorites originated from Mars are as follows: (1)SNC meteorites (except ALH8400 1) have a very small crystallization age (≤ 1.3Ga), which can't be formed by the igneous action of asteroids. 1.3Ga is similar to the age of Mars Tarslich volcano (Wood (2) Isotopic compositions of CO2, N2 and rare gases captured in EETA 79001meteorite glass, and the values of 13C and 12C are consistent with the Martian atmosphere (Bogard & Johnson,1983); (3) Hydrous silicate minerals such as iddingsite and amphibole have been found in some meteorites, titanium amphibole has been found in magma inclusions of ALHA77005 (Ikeda, 1998), and idding site(Reid & Bunch,1975) has been found in Nakhla; (4) Extraterrestrial carbonates (Romanek et al., 1994, 1995) have been found in some meteorites (such as ALH 8400 1), but it is known that lunar igneous rocks and asteroid igneous rocks actually do not contain carbonates; (5) The results of X-ray fluorescence spectrum analysis of Martian soil by Viking and Mars Pathfinder are quite consistent with the chemical composition of the whole rock of Sergo Ti meteorite, especially their FeO content is almost the same (19.7% and19.6% respectively); (6) Phosphate with high fO2, special18o/16o (△17o ≈ 0.3% o) (Clayton & Mayeda, 1996) and high D/H ratio (Leshin et al. Lessing, 2000; Waston et al., 1994), a complex REE distribution pattern, and similar ratios of FeO/MnO, K/La, K/U, W/La, Ga/La and Na/Al, have higher volatile element content than other chondrites (Wang Daode,1995; Wang Daode et al., 1999).
(3) Petrological characteristics of Martian meteorites
Table 28-4 lists the petrological and mineralogical characteristics of four kinds of Martian meteorites. Table 28-5 lists the mineral type abundance (volume percentage) of Martian meteorites. Fig. 28- 17 shows the micrograph of olivine chondrite under single polarization, and fig. 28- 18 shows the X-ray scanning of Fe element in flash chondrite.
Table 28-4 Petrological Characteristics of Martian Meteorites
Table 28-5 Summary of Abundance of Martian Meteorite Mineral Model (WB/%)
(According to Meyer C., 2006)
Fig. 28- 17 spherulite characteristics of olivine under single polarization.
Pyroxene and olivine are the main mineral phases of Martian meteorites. The composition of pyroxene in pyroxene chondrite varies greatly, and FeO content is high. Olivine is the main mineral phase in pure olivine chondrite. Compared with olivine in other Martian meteorites, Fa value is the lowest. Almost all feldspar in Martian meteorites is transformed into molten feldspar by impact melting. The chemical composition of molten feldspar from S, N to C tends to be rich in K and Na and poor in Ca. Chromite and magnetite are the main opaque minerals of Martian meteorites. Martian meteorites are totally crystalline, with different fire compositions (Figure 28- 19), much like basalt and diabase on the earth.
GRV 99027 Martian meteorite has a medium-sized fully crystalline grain structure (Figure 28-20). It is mainly composed of olivine and pyroxene, with a small amount of molten feldspar, and the auxiliary minerals are chromite, pyrrhotite and phosphate. It can be seen that GRV 99027 is a chondrite with typical characteristics of different fire compositions, including three basic structures: metamorphic structure, non-metamorphic structure and fused bag structure (Figure 28-20).
Fig. 28- 18 x-ray scanning of iron in pyroxene chondrite slices.
The study of SNC Martian meteorites provides abundant information on meteorite petrography and trace elements and isotopes of meteorites. These are not available through remote sensing of Mars.
SNC Martian meteorites, except ALH8400 1, are generally believed to originate from young craters on Mars, which reflects that sputtering samples on the Martian crust are biased towards young magmatic rocks. Ancient shells may be too easily crushed to sputter and escape from Mars (McSween et al., 2002). Photographs taken by Mars spacecraft show that layered sediments are common on Mars and are sometimes interpreted as sedimentary rocks (Malin &Edgett, 2000). But strangely, there are no sedimentary rocks and no magmatic rocks with andesite composition in Martian meteorites. However, salts and clay minerals in some Martian meteorites show that rocks on or near the surface of Mars have been transformed due to the interaction of fluid or seawater (Bridges et al., 200 1). The subsequent Mars Global Surveyor (MGS) used TES spectrum to analyze the data (Bandfield et al., 2000; Hamilton et al., 200 1) shows that the plains in the northern hemisphere of Mars are mainly andesite, and the total alkali-SiO2 _ 2 analysis diagram (Figure 28-2 1) shows that the TES spectral data (Surface _ 2) is consistent with the composition of the fireless stardust of Mars pathfinder. The surface 1 and Martian meteorite composition show basaltic composition. It is difficult to explain the andesite that formed the Martian hemisphere, especially because there is no plate subduction on Mars. Although water-bearing magma only needs low differentiation, it is impossible for basaltic magma differentiation to produce andesite melt. Some evidences show that the magma of gabbro chondrites contains a certain amount of water before eruption (McSween et al., 200 1), but this hypothesis is still controversial.
Fig. 28- 19 Dag 476 olivine-olivine backscattered electron image is a typical heterogeneous structure of fire composition. Metamorphic pyroxene (Pgt) and fused feldspar (Msk) constitute a typical basalt structure, and olivine (Ol) is embedded in basalt phase in the form of phenocrysts. Other mineral phases: Chr is chromite; It's meteorite, sulfur and iron.
Figure 28-20 backscattered electron image 28-20 GRV 99027 Martian meteorite
The geochemical characteristics of Martian meteorites reflect the characteristics of the source region of Martian mantle, but in the process of magma rising and emplacement, its composition will change due to the influence of separation and crystallization and sometimes assimilation and contamination. SNC Martian meteorites are rich in Mn and P, and do not lose Fe and other iron-loving elements. However, compared with the rocks on earth, these meteorites are relatively poor in Al, which is reflected in the Martian soil and the rocks without fire dust detected by Mars Pathfinder.
Fig. 28-2 1 Chemical Classification Diagram of Volcanic Rock Samples from Mars
(4) The research content and significance of Martian meteorites.
At present, the research contents of Martian meteorites mainly include: (1) classification, rock mineralogy, chemical composition, magnetism and spectral characteristics of Martian meteorites; (2) Material source, melting differentiation, parent magma composition, thermal metamorphism, impact effect, radiation record and effect, secondary alteration, etc. Mars meteorite. (3) Radioactivity, cosmogenic isotopic composition and chronology, stable isotopes (H, O, S, C, N) and other isotopic compositions (Xe, W, Hf, Re-OS); (4) Comparative study of Martian meteorites with earth rocks, moon meteorites and other types of meteorites; (5) Composition, structure, properties and magmatic activity of Mars crust-mantle-core, composition and evolution of Mars atmosphere, and interaction among Mars atmosphere, hydrosphere and lithosphere; (6) Possible life remains on Mars, etc.
At present, the main problems existing in the international study of Martian meteorites are: (1) At present, there are no rock samples directly taken from Mars. Only by studying Martian meteorites can we reveal the material composition, structure and magma evolution law of Mars. The number of Martian meteorites recovered by people is very small, and most of them are piled rocks. Some Martian meteorites with the same rock type have basically the same sputtering age (N meteorite is about 1 1Ma, and L-S meteorite is about 4Ma), which is probably the product of the same impact event. Therefore, there are very few Martian surface areas represented by Martian meteorites, which limits people's understanding of Martian material composition, parent magma composition and evolution law. (2) Using low-temperature altered minerals (such as hydrous minerals, clay minerals, carbonates, sulfates, etc.). Studying the hydrothermal system and alteration on the surface of Mars in Martian meteorites can provide clues for the evolution of Martian atmosphere, but at present, almost all the work is focused on ALH 8400 1, and there is no corresponding systematic and in-depth study on the secondary minerals of other Martian meteorites. (3) Mars meteorites are sputtered from Mars and need to overcome the strong gravity of Mars when landing on the earth. Therefore, most Martian meteorites have experienced very strong impact metamorphism, so it is necessary to deduct the impact metamorphism superimposed on magmatism in the study of Martian meteorites. However, at present, the research on the impact effect of Martian meteorites is mainly limited to a few meteorites, and further work is needed to study the impact metamorphism on the surface of Mars. (4) Isotopic dating provides a time scale for the formation and evolution of Martian meteorites. Some isotopic systems (such as K-Ar, Rb-Sr, etc. ) was reset due to strong impact metamorphism, which led to a long-term dispute on the interpretation of isotopic age. (5) The debate about the existence of life on Mars is the focus of current Martian meteorite research and Mars exploration. There are evidences of ancient life traces in ALH 8400 1 meteorite, such as low-temperature carbonate tumor, biofilm and polycyclic aromatic hydrocarbons, but the experimental simulation of hot water solution (Golden et al., 2000; McSween et al., 1998) observed that inorganic carbonate precipitation of magnesium and calcium can produce carbonate tumor of ALH8400 1. (6) The study of Martian meteorite reflection spectrum will be of great reference value for correctly interpreting a large number of remote sensing data of Mars probes and spacecraft.